1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus and more particularly, to an apparatus in which a sheet whose first surface is formed with an image by an image forming section is re-fed to the image forming section for forming an image on the second surface of the sheet which is opposite side from the first surface.
2. Description of the Related Art
There is known image forming apparatuses such as copiers, printers and facsimile machines in which an image is formed on the first surface of a sheet by an image forming section and an image is formed on the second surface of the sheet which is opposite side of the sheet.
In order to form images on both surfaces of a sheet, such an image forming apparatus includes re-feeding means which, after an image is formed on one surface of the sheet, again supplies the sheet to the image forming section to form an image on the back surface.
A configuration of the conventional re-feeding means will be explained based on FIGS. 8 to 11.
After an image is formed on the first surface of a sheet S, the sheet S is turned over, the sheet passes through a re-feeding passage 18, the sheet is fed by a pair of obliquely conveying rollers 11a and 11b, and the sheet is fed to the image forming section again. The pair of obliquely conveying rollers comprise an obliquely conveying roller 11a and an obliquely conveying follower roller 11b which comes into contact with the obliquely conveying roller 11a under pressure. In the image forming section, the second surface of the sheet is formed with an image.
In such a configuration, when the sheet S is re-fed to the image forming section, the sheet S is inclined during the conveying operation and deviated from a conveying-reference, and when an image is formed on the second surface, the sheet S and the image are deviated from each other in some cases. This is because that when an image is formed on the second surface, a conveying path of the sheet required until an image is formed on the second surface of the sheet is longer than a case in which an image is formed on the first surface and thus, the sheet is slightly deviated from the conveying-reference during the conveying operation due to eccentricity of the rollers, a difference in pressure or a different in resistance of a conveying surface.
In order to prevent the sheet S from being deviated, it is necessary to adjust the position of the sheet S in its widthwise direction after an image is formed on the first surface and before an image is formed on the second surface, and to adjust the position of the sheet S such that the image and the sheet S coincide with each other. To adjust the position, there is a method in which during the conveying operation of a sheet, the sheet is fed while pushing the sheet against a reference member which functions as a positioning reference disposed on an end of the re-feeding passage 18, thereby correcting the inclination and to align the sheet with the conveying-reference.
FIG. 8 is a plan view showing a configuration of the re-feeding passage 18 having the re-feeding means, FIG. 9A is a front view thereof and FIG. 9B is a side sectional view thereof.
In FIGS. 8 and 9, a reference number 13 represents roller shafts for two obliquely conveying rollers 11a. As shown in FIG. 9A, the roller shafts 13 are held by bearings 14 such that the roller shafts 13 can rotate with respect to a reference guide 12 which is a reference member.
Pulleys 15 are fixed to ends of the two roller shafts 13. The roller shafts 13 are driven by belts 16a and 16b which are wound around the pulleys 15. The obliquely conveying rollers are driven by a driving motor (not shown) through the belts 16a and 16b. 
A reference number 17 represents pins which rotatably holds obliquely conveying follower rollers 11b. The pins 17 are held by a conveying upper guide 19 which forms an upper surface (ceiling surface) of the re-feeding passage 18. A reference number 181 represents springs which push the pins 17 from above. The obliquely conveying follower rollers 11b come into contact with the obliquely conveying rollers 11a by the springs 181 under given pressure.
Here, the re-feeding passage 18 is a so-called one side reference type in which one side of a sheet S is pushed against the reference guide 12 to adjust a position of the side end of the sheet S. The reference guide 12 is provided with a plurality of, e.g., first and second reference guide surfaces 12a and 12b. A reference number 20 represents conveying lower guide forming a lower surface of the re-feeding passage 18 together with the reference guide 12.
In the re-feeding passage 18 which adjust a position of the sheet S by the one side reference, the sheet S fed in the direction of arrow A in FIG. 8 is obliquely fed by the obliquely conveying rollers 11a and the obliquely conveying follower rollers 11b. 
The obliquely conveying rollers 11a and the obliquely conveying follower rollers 11b are inclined at given angle so that a force in the direction of the reference guide surface is applied to the sheet S so as to push the sheet S against the reference guide surfaces 12a and 12b. With this configuration, the sheet S is fed while changing its orientation toward the reference guide surfaces, and the end of the sheet S is pushed against the reference guide surfaces 12a and 12b, thereby correcting the inclination of the sheet, and the position of the sheet S is adjusted with respect to the conveying-reference.
In order to adjust an attitude of the sheet S, if the force of the pair of obliquely conveying rollers 11a and 11b in the direction of the reference guide surface which pushes the side end of the sheet S against the reference guide surfaces 12a and 12b and conveys the sheet S along the reference guide surfaces 12a and 12b is sufficient, one of the obliquely conveying roller 11a and the obliquely conveying follower roller 11b may be inclined at a given angle.
Thus, the first and second reference guide surfaces 12a and 12b against which the side end of the sheet S is abutted is determined by a length of the sheet S in its widthwise direction. For example, the first reference guide surface 12a has side end reference of sheet S of letter size and A4 size, and the second reference guide surface 12b has side end reference of sheet S of executive size and B5 size.
In the case of a sheet of A4 size whose widthwise length is about 6 mm shorter than that of the letter size, since the position is adjusted by the same first reference guide surface 12a as that used for a sheet of letter size, it is necessary to correct such that an image is formed on the second surface while deviating the image by 3 mm in the lateral direction as compared with a case in which an image is formed on a letter size sheet. For the executive size sheet and B5 size sheet also, the same correction is necessary.
FIG. 10 show the re-feeding passage 18 capable of conveying sheet S1 of A5 size which is smaller than the above sheet S. Since this re-feeding passage 18 conveys the sheet S1 of A5 size, the re-feeding passage 18 is provided with a reference guide 12 having a third reference guide surface 12c so as to fit the sheet S1 having smaller width.
When such a smaller sheet S1 is supplied again, since the length of the sheet S1 in the sheet conveying direction is short, the length of the sheet S1 becomes shorter than distances between a conveying roller 3g shown in FIG. 7 and the pair of upstream obliquely conveying rollers 11a and 11b, and it becomes difficult to convey the small sheet S1.
When the small sheet S1 is to be fed, auxiliary rollers 11a′ shown in FIG. 10 are added at upstream of the pair of obliquely conveying rollers 11a and 11b so that the small sheet S1 can be fed. Since the auxiliary rollers 11a′ are added, a belt 16c is also added so that three roller shafts 13 can be driven.
As shown in FIG. 11A, the reference guide surfaces 12a, 12b and 12c respectively having different size are inclined stepwisely in the widthwise direction of a sheet, a support surface 12a1 horizontally extends from a lower end of the first reference guide surface 12a and supports one end of letter size sheet and A4 size sheet. The support surface 12a1 is located higher than a guide surface 20a of a conveying lower guide 20 which guides the other end of the sheet.
With this configuration, a height difference exists between the reference guide 12 and the conveying lower guide 20. When such a height difference exists, if A4 size sheet S is fed into a paper re-feed unit in the direction of arrow B as shown in FIG. 11B, a difference is generated in a paper-pass length in the conveying direction, and due to this difference in paper-pass length, the sheet S is inclined.
If the sheet S is inclined at upstream of the pair of obliquely conveying rollers 11a and 11b, the tip end corner of the sheet S closer to the reference guide surface is separated from the first reference guide surface 12a and fed.
Even if the tip end corner of the sheet S closer to the reference guide surface is separated from the first reference guide surface 12a in this manner, if the sheet S is nipped between the pair of obliquely conveying rollers 11a and 11b thereafter, since a force for moving the sheet S toward the reference guide surface is usually applied to the sheet S, when the sheet S is to be discharged out from the paper re-feed unit, the sheet S is discharged in a state in which the side end of the sheet is along the first reference guide surface 12a. 
However, if the height difference is excessively large, the pair of obliquely conveying rollers 11a and 11b can not move the sheet S toward the reference guide surface in some cases. In such a case, the precision of image forming position of the second surface is adversely affected.